1. Field of the Invention.
This invention relates to a rotary seal for a directional drilling tool, which is useful when drilling boreholes into the earth.
2. Description of the Related Art.
When drilling boreholes into sub-surface formations, it is desirable to be able to vary the direction of drilling. For example to direct the borehole toward a desirable target or to control the direction, a rotary steerable tool may be used in a drill string to change the direction of the borehole. It may also be desirable to correct for deviations from the desired direction when drilling a straight hole. Furthermore, directional drilling tools may also be used to control the direction of the hole to avoid obstacles.
Typical rotary steerable tools are shown for example in U.S. Pat. Nos. 5,603,385; 5,520,255 and 5,706,905 all herein incorporated by reference for all they disclose. Typically, bias units within these rotary steerable tools comprise a number of hydraulic actuators spaced apart around the periphery of the unit. Each actuator may have a movable thrust member which is hydraulically displaced outwardly for engagement with the formation of the borehole being drilled. Typically, each actuator also has an inlet passage for connection to a source of drilling fluid under pressure and an outlet passage communicating the annulus of the borehole. A selector control valve connects the inlet passages in succession to the source of fluid under pressure as the bias unit rotates. This modulates the fluid pressure supplied to each actuator in synchronism with the rotation of the drill bit and in selective phase relation thereto. As a result, as the drill bit rotates, each movable thrust member is displaced outwardly at the same rotational position so as to bias the drill laterally and therefore control the direction of drilling.
Rotary steerable tools are typically designed as a single unit comprising a selector control valve assembly, a plurality of actuators, and logic systems to control these actuators. In operation, however, because power to actuate the valve assembly is limited, the valves have been known to stick. One probable reason for these valves sticking is the force applied to the valve by the differential pressure of the drilling fluid. Particularly, when the valve is initially actuated, this hold down force can require the starting torque of the valve to be higher than the torque available in the actuator. In the past, this condition has caused failure of the tool and an expensive trip of the drill string from the hole.
It was initially believed that the valves became stuck due to a tribological buildup between the valve faces as described in U.S. Pat. No. 5,560,716 incorporated herein by reference for all it discloses. This patent describes a tribological buildup of the cobalt binder material on the faces of heavily loaded polycrystalline diamond bearing elements, leading to an increase in the operating torque. The patent teaches that this buildup can be alleviated by an acid wipe. However, since the sticking often occurred on initial startup, there was no relative rotation of the valve bodies under load to cause this tribological buildup, so any benefit from this treatment was at best only partially effective. Therefore, it is now believed that other factors independent of the valve material composition contribute to problem, as will be described.
Another problem with these prior art valve assemblies is that during operation, pressure surges in the drilling fluid can suddenly increase the load on the valve such that the operating torque of the valve exceeds the torque available by the actuator. It is believed that this is caused by the hydrostatic pressure being exerted across the relatively smooth surface of these selector valves exceeding the hydrodynamic lubrication effect of the fluid.
An additional problem with these units has been a sudden reverse flow of drilling fluid through the valve assembly caused under conditions when the source of pressurized fluid at the surface has been suddenly stopped or when one or more of the actuators are rapidly forced closed. This has been known to cause the valve surfaces to become separated. Since the valve design relies upon the differential pressure of the drilling fluid to provide sealing force, when the valve faces become separated, there is sometimes not enough differential pressure to force the valve surfaces back into contact.